Custom Normals 1: "Flat" Face

The simplest way to apply custom normals to a face is to flatten them, giving the face the appearance that it is a flat surface that is uniformly lit.

Select your head/face mesh and open the right-side panel (press N). Under the Abnormal tab, there is a drop down selector that should show a list of your mesh's vertex groups. Select the "Custom Normals" vertex group we created earlier.

Start Abnormal with the "Start Normal Editor" button. Every vertex should already be selected, and you'll see long pink lines in every direction representing the directions of the normals.

Select "Sphereize Normals" and then "Confirm Sphereize" to apply. The normals of the face will appear spherical. Under "Flatten Normals on Axis", press both the X and Z buttons. This flattens the normals on the X and Z axis, meaning they are facing directly forward. "Confirm Changes" under the "Finish Modal" panel.

The shading of your face should now appear completely flat. This already looks pretty good in most cases, but we can do more.